Effects of Ca²⁺, Zn²⁺ and Cd<2+> on Uridine Diphosphate-Glucuronyltransferase and β-Glucuronidase Activities in Rat Liver Microsomes

Abstract

The effect of various metals on uridine diphosphate (UDP)-glucuronyltransferase and β-glucuronidase activities in rat liver microsomes was investigated. The presence of Mn²⁺, Cd²⁺, Zn²⁺, V⁵⁺, Ni²⁺, Co²⁺, Cu⁺ or Ca²⁺ (20μM) in the enzyme reaction mixture did not a significant alteration of UDP-glucuronyltransferase activity in hepatic microsomes. Of these metals, Zn²⁺ and Cd²⁺ (20μM) caused a remarkable increase in hepatic microsomal β-glucuronidase activity. Appreciable effects of Zn²⁺ and Cd²⁺ on β-glucuronidase activity were seen at 5.0 μM, and the effects were saturated at 50 μM. Ca²⁺ (5.0-50 μM) and/or the Ca²⁺-binding protein regucalcin (2.0μM) did not have an appreciable effect on UDP-glucuronyltransferase and β-glucuronidase activities in hepatic microsomes. Thus, Zn²⁺ and Cd²⁺ uniquely increased β-glucuronidase activity. The Zn²⁺-and Cd²⁺-induced increase in β-glucuronidase activity was completely reversed by the presence of an SH group-protecting reagent (dithiothreitl). The response of the microsomal enzyme to Zn²⁺ and Cd²⁺ (20 μM) was no longer seen after treatment with 0.2% Triton X-100 [polyoxyethylene(10) octylphenyl ether], indicating that the stimulation by these metals is dependent on membrane association. The present study suggests that, of various metals tested, Zn²⁺ and Cd²⁺ can uniquely increase hepatic microsomal β-glucuronidase actibity, and that their effect is based on binding to membranous SH groups, beside the enzyme protein.